Probe holder for low current measurements

ABSTRACT

A system for low-current testing of a test device includes a probing device for probing a probing site on the test device. The probing device includes a dielectric substrate having first and second sides, an elongate conductive path on the first side of the substrate, an elongate probing element connected to the elongate conductive path so as to extend in a cantilevered manner beyond the substrate, and a conductive area on the second side of the substrate. The probe housing is matingly detachably engageable with the probing device.

[0001] The present invention relates to a probe holder suitable for lowcurrent measurements.

[0002] Typically, in the construction of a probe card, a dielectricboard is used as a base. A plurality of probing devices are mounted in aradial arrangement about an opening in the board so that the probingelements of these devices, which may, for example comprise slenderconductive needles, terminate below the opening in a pattern suitablefor probing the contact sites of the test device. The probing devicesare individually connected to the respective channels of a testinstrument by a plurality of interconnecting lines, where the portion ofeach line that extends between the corresponding probing device and theouter edge of the dielectric board may comprise an interconnecting cableor a conductive trace pattern formed directly on the board. In oneconventional type of setup where the test devices are integratedcircuits formed on a semiconductor wafer, the probe card is mounted by asupporting rig or test head above the wafer, and a support beneath thewafer moves the wafer so that each device thereon is consecutivelybrought into contact with the needles or probing elements of the probecard.

[0003] With particular regard to probe cards that are specially adaptedfor use in measuring ultra-low current (down to the femtoamp region orlower), probe card designers have been concerned with developingtechniques for eliminating or at least reducing the effects of leakagecurrents, which are unwanted currents that can flow into a particularcable or channel from surrounding cables or channels so as to distortthe current measured in that particular cable or channel. For a givenpotential difference between two spaced apart conductors, the amount ofleakage current that will flow between them will vary depending upon thevolume resistivity of the insulating material that separates theconductors, that is, if a relatively lower-resistance insulator is used,this will result in a relatively higher leakage current. Thus, adesigner of low-current probe cards will normally avoid the use ofrubber-insulated single-core wires on a glass-epoxy board since rubberand glass-epoxy materials are known to be relatively low-resistanceinsulators through which relatively large leakage currents can flow.

[0004] One technique that has been used for suppressing interchannelleakage currents is surrounding the inner core of each lead-in wire witha cylindrical “guard” conductor, where the “guard” conductor ismaintained at the same potential as the inner core by a feedback circuitin the output channel of the test instrument. Because the voltagepotentials of the outer guard conductor and the inner conductive coreare made to substantially track each other, negligible leakage currentwill flow across the inner dielectric that separates these conductorsregardless of whether the inner dielectric is made of a low- orhigh-resistivity material. Although leakage current can still flowbetween the guard conductors of the respective cables, this is typicallynot a problem because these guard conductors, unlike the innerconductive cores, are at low impedance. By using this guardingtechnique, significant improvement may be realized in the low-levelcurrent measuring capability of certain probe card designs.

[0005] To further improve low-current measurement capability, probecards have been constructed so as to minimize leakage current betweenthe individual probing devices which mount the probing needles or otherelements. With respect to these devices, higher-resistance insulatingmaterials have been substituted for lower resistance materials andadditional conductive surfaces have been arranged about each device inorder to perform a guarding function in relation thereto. In one type ofassembly, for example, each probing device is constructed using a thinblade of ceramic material, which is a material known to have arelatively high volume resistivity. An elongate conductive trace isprovided on one side of the blade to form the signal line and abackplane conductive surface is provided on the other side of the bladefor guarding purposes. The probing element of this device is formed by aslender conductive needle, such as of tungsten, which extends in acantilevered manner away from the signal trace. Such devices arecommercially available, for example, from Cerprobe Corporation based inTempe, Ariz. During assembly of the probe card, the ceramic blades areedge-mounted in a radial arrangement about the opening in the card sothat the needles terminate within the opening in a pattern suitable forprobing the test device. The conductive backplane on each blade isconnected to the guard conductor of the corresponding cable and also thecorresponding conductive pad or “land” adjacent the opening in the card.In this manner each conductive path is guarded by the backplaneconductor on the opposite side of the blade and by the conductive landbeneath it.

[0006] It has been found, however, that even with the use of guardedcables and ceramic probing devices of the type just described, the levelof undesired background current is still not sufficiently reduced as tomatch the capabilities of the latest generation of commerciallyavailable test instruments, which instruments are able to monitorcurrents down to one femtoamp or less. Thus, it was evident that otherchanges in probe card design were needed in order to keep up with thetechnology of the latest test instrument design.

[0007] However, in the design of such probe cards the ceramic blades arepermanently mounted to the probe card and thus when damaged the entireprobe card may need to be replaced or the damaged ceramic blade somehowrepaired at substantial expense and effort. Referring to FIG. 1, inorder to provide probe tips that are more easily replaced, a probehousing 10 with a replaceable probe tip 12 was designed. A pair oftriaxial cables (not shown), each of which includes a shield, a guard,and a signal conductor, extend from measurement equipment (not shown) toa location within a chamber (not shown) that encloses the probe tip 12,the probe housing 10, and the test device. Each triaxial cable isconnected to a respective coaxial cable 14 and 16 that includes a guardand a signal conductor. The shield conductor of each of the triaxialcables may be connected to the chamber, if desired. The chamberenvironment is shielded so it is unnecessary to include the shieldconductors all the way to the probe housing 10. In addition, the probehousing 10 includes relatively small connectors which are much moresuitable for connection to relatively small coaxial cables 14 and 16, asopposed to relatively large triaxial cables. The probe housing 10includes a pair of connectors 18 and 20, each of which provides aconnection to a respective one of the coaxial cables 14 and 16. Theguard of each of the coaxial cables 14 and 16 is electrically connectedto the conductive exterior of the probe housing 10, which reduces thecapacitance and leakage currents to the probe tip 12.

[0008] Ideally in a two lead coaxial cable system a “true Kelvin”connection is constructed, although not shown in FIG. 1. This involvesusing what is generally referred to as a force signal and a sensesignal. The signal conductor from one of the coaxial cables isconsidered the force conductor, while the signal conductor from theother coaxial cable is considered the sense conductor. The forceconductor is brought into contact with a test pad on the wafer. Theforce conductor is a low impedance connection, so a current is forcedthrough the force conductor for testing purposes. The sense conductor isa high impedance connection and is also brought into contact with thesame test pad on the wafer, preferably in close proximity to the senseconductor, in order to sense the voltage. As such the current versusvoltage characteristics of the test device can be obtained using theforce and sense conductors.

[0009] To calibrate the “true Kelvin” connection, first an open circuittest is performed to measure the capacitance without the test padcapacitance. This is performed by picking up the probe and shorting theprobe tips of the sense and force conductors together with bothsuspended in air. The open circuit test is difficult to perform. Second,a short circuit test is performed to measure the capacitance when theforce and sense conductor tips are on the test pad. From the opencircuit test and the short circuit test the cable impedance is obtainedand thereafter used for offsetting during subsequent measurements.Unfortunately, calibration of a “true Kelvin” connection is difficultand time consuming to perform. Additionally, the current flowing throughthe force conductor is generally known but the resistance drop along thelength of force conductor results in the exact voltage at its end to beunknown, therefore the measurement can be inaccurate. Further, the testpads on the test device are normally small, in order to minimize cost,which makes it difficult to position two needles on the test pad.Furthermore, using two needles requires additional space for the needlesand supporting structure that may not be available when a large numberof probe needles are simultaneously necessary to test a small area ofthe test device, such as a silicon wafer.

[0010] Referring again to FIG. 1, to permit the use of a single probetip, which permits more tests to be simultaneously performed in aconfined area, the force conductor 22 and the sense conductor 24 areelectrically connected together with a combined conductor 26 within theprobe housing 10. Coaxial cable 14 would be the force connection whilecable 16 would be the sense connection. The guard conductor of the forcecable 14 and the guard conductor of the sense cable 16 are electricallyconnected to the conductive probe housing 10. The combined force andsense conductor 26 is electrically connected to a probe connector 28 atone end of the probe housing 10. A rigid coaxial probe tip cable 30 isdetachably connected to the probe connector 28. The rigid coaxial probetip cable 30 includes both a copper guard conductor 32 plated with goldand a central signal conductor 34 made of tungsten. The guard conductor32 of the rigid coaxial probe tip cable 30 is electrically connected tothe probe housing 10, which is in turn connected to the guard conductorsof the coaxial cables 14 and 16. The length of the signal path extendingfrom the point that the force conductor 22 and sense conductor 24 areconnected together carries current during measurements which results ina voltage drop from any internal resistance in that portion of thesignal path. The assumption is that for low current applications, thevoltage drop due to the resistance is small because the junction isclose to the probe tip 12 and the conductor has low resistance. However,the rigid coaxial probe tip cable 30 is difficult to replace ifdefective or damaged during use. The test device shown in FIG. 1,provides reasonably accurate low current measurements. Unfortunately, itwas observed that the device shown in FIG. 1, when used over a widerange of temperatures, such as −65 degrees celsius to 300 degreecelsius, results in unacceptable levels of noise. For example, in onecommercial embodiment, noise in the range of +−100 femtoamps wasobserved over only a temperature range from room temperature to 150degrees celsius. As previously mentioned, modern measurement instrumentsare capable of measuring much lower current levels and thus such noiselevels obscures low current measurement levels under 10 femtoamps.

[0011] What is desired, therefore, is a low current measurement devicethat has substantially lower noise levels. In addition, such ameasurement device should be provide for easy replacement of probe tips.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a sectional view of a conventional probe housing andcable type probing device.

[0013]FIG. 2 is a pictorial view of an exemplary embodiment of a probehousing with a probe connector, and probing device of the presentinvention.

[0014]FIG. 3 is a sectional view of the probe housing and probing deviceengaged within the probe connector of FIG. 1.

[0015]FIG. 4 is a sectional view of the probe connector and probingdevice of FIG. 1.

[0016]FIG. 5 is an exemplary low-noise cable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] After observing excessive noise levels over wide temperatureranges resulting from the device shown in FIG. 1, the present inventorcame to the startling realization that a Teflon insulator 36 between thesignal conductor 34 and the guard conductor 32 of the coaxial probe tipcable 30 expands and contracts when the temperature is increased anddecreased, respectfully. The expansion and contraction of the Tefloninsulator 36 results in relative movement and friction with respect tothe signal conductor 34 and the guard conductor 32. The relativemovement generates electrical charges between both the guard conductor32 and the signal conductor 34 in contact with the Teflon insulator 36due to friction. Free electrons rub off the signal conductor 32 and theguard conductor 34 which creates a charge imbalance resulting in acurrent flow.

[0018] Triboelectric currents are traditionally considered to arise inrelation to the physical bending of materials, such as coaxial cables.The time during which the coaxial cable undergoes physical bending isrelatively long, e.g. low frequency, and does not significantlycontribute to the triboelectric current generation. The present inventorbelieves that instead of the physical bending resulting in the majorsource of the generation of triboelectric currents, it is in fact a muchmore subtle phenomena involving the slight contraction and expansion ofthe Teflon insulator that creates the frictional movement previouslydescribed, and hence the undesired triboelectric current generation.

[0019] With the identification of the source of the noise, which had notbeen previously identified by previous probe card designers, the presentinventor developed ingenious solutions to the identified problem.

[0020] In order to reduce the triboelectric current generation, thecable 30 is replaced with “low-noise” cable. “Low-noise” cable greatlyreduces triboelectric currents by typically using an inner insulator ofpolyethylene coated with graphite. The graphite provides lubrication anda conduction equipotential cylinder to equalize charges and minimizecharges generated by frictional effects of cable movement.

[0021] It will be noted that the inventor does not claim to havediscovered a new solution to the problem of the triboelectric effect asembodied by the use of “low-noise” cable. A relatively straightforwardsolution to this problem can be found in the field of cable technologywherein it is known how to construct a “low-noise” cable by using anadditional layer of material between the outer conductor and the innerinsulator, which material is of suitable composition for suppressing thetriboelectric effect. This layer, in particular, includes a nonmetallicportion that is physically compatible with the inner insulator so as tobe prevented from rubbing excessively against this dielectric and, onthe other hand, includes a portion that is sufficiently conductive thatit will immediately dissipate any charge imbalance that may be createdby free electrons that have rubbed off the conductor. It is not claimedby the inventor that this particular solution to the triboelectriceffect problem is his invention. Rather it is the recognition that thisspecific problem is a major source of performance degradation in thefield of low-current probe station design, and in particular degradationwhen testing over a range of temperatures, that the inventor regards ashis discovery.

[0022] As previously described, one embodiment of a probe holder designof the present invention includes the replacement of cable 30 with a“low-current” cable that includes conductive and dielectric layers in acoaxial arrangement with each other and further includes at least onelayer of material within each cable adapted for suppressing thetriboelectric effect so as to minimize any undesirable currents thatwould otherwise be generated internally in each cable due to thiseffect. This layer of material on the probe holder enables the probestation to be used for the measurement of ultra-low currents even over arange of temperatures.

[0023] In the field of radio frequency (rf) cable technology, cablesthat include a layers material of the type just described are generallyreferred to as “low-noise” cables. Commercial sources for this type ofcable include Belden Wire and Cable Company based in Richmond, Ind.,Suhner HR-Kabel based in Herisau, Switzerland, and Times MicrowaveSystems based in Wallingford, Conn.

[0024] While the replacement of the cable 30 with “low-noise” cablesignificantly enhances the low-noise characteristics of the device 10,the cable is expensive to obtain in small quantities, awkward to replacein a confined environment if damaged, and the precise bending of thecable to the test pad together with the precise location of the probetip is difficult to control with a cable type connector. Even moreimportantly, the Teflon insulator material in coaxial cables aresusceptible to “cold flow” out of the end of the cable when subjected tosignificant temperatures. The resulting cable insulator will be thinnerin portions thereby changing the characteristics of the cable over time.

[0025] Referring to FIG. 2, in order to overcome the limitations ofcable type connectors, an alternative embodiment includes a conductiveprobe housing 50, similar in structure to the probe housing 10 shown inFIG. 1, with an elongate probe connector 52. The probe connector 52 isconductive and preferably has a rectangular cross section. An insert 54is sized to fit within the probe connector 52. The insert 52 includes aceramic insulator 56 and a conductive bent connector 58 attached to oneside of the insulator 56. The insulator 56 is in face-to-face abutmentwith the interior upright surface 59 of the probe connector 52. A“blade” type probe 60, as described in the background, is matinglydetachably engageable within the probe connector 52.

[0026] Referring also to FIG. 3, the blade 60 preferably includes adielectric substrate 62 formed of a ceramic or a comparablehigh-resistance insulating material. The blade 60 has a pair of broadparallel sides or faces interconnected by a thin edge. Formed on oneside of the blade 60 is an elongate conductive path 64, while the otherside includes a backplane conductive surface 66. A needle 68 issupported by the dielectric substrate 62 and electrically connected tothe elongate conductive path 64. In the particular embodiment shown, theblade 60 is generally L-shaped in profile and is edge-mounted within theprobe connector 52 so that the short arm of the L-shaped blade 60extends downwardly making contact with the test device. As previouslyindicated, blades 60 having a construction of the type just describedare commercially available from Cerprobe Corporation of Tempe, Ariz.

[0027] Referring also to FIG. , when the blade 60 is slidably engagedwithin the probe connector 52, the back-plane conductive surface 66 isin face-to-face contact with the inner upright surface 59 of the probeconnector 52. Accordingly, a ground signal path is provided from theguard conductors of the force and sense cables 14 and 16, though theprobe housing 50 and probe connector 52 to the backplane conductive area66 of the blade 60. This provides a ground path to a location near theend of the needle 68. In addition, a conductive path is provided fromforce and sense conductors 72 and 74 connected to cables 14 and 16,through a combined conductor 70 to the bent connector 58. It is to beunderstood that the combined connector 70 may be any suitable type ofcoupler that electrically connects the force and sense cables to theconductive path 64 on the blade 60. Likewise it is to be understood thatthe electrical connection between the backplane 66 on the blade 60 andthe cables 14 and 16 may be any suitable type of coupler. The bentconnector 58 is resiliently deformable as the blade 60 is inserted intothe probe connector 52 and exerts pressure between the backplaneconductive surface 66 and the upright surface 59 so a low lossconnection is maintained. Also the pressure maintains the position ofthe blade 60 during use. Simultaneously, the bent connector 58 exertspressure between the conductive path 64 and the bent connector 58 toprovide a low loss connection. A signal path is thus provided throughthe needle 68, the conductive path 64, the bent connector 58, and thecombined conductor 70 to the force conductor 72 and sense conductor 74.

[0028] The probe embodiment shown in FIGS. 2-4 does not includestructure that is subject to the generation of triboelectric currentsand further is free from materials that deform over a normal range oftemperatures used during probing. In addition, the blades 60 are readilyreplaceable if damaged during use.

[0029] While the preferred embodiment of the present invention embodiesa set of two cables 14 and 16, with one including the sense conductorand the other including the force conductor, it is to be understood thatthe present invention also encompasses a probe holder with a singlecoaxial, or triaxial, cable.

[0030] The terms and expressions which have been employed in theforegoing specification are used therein as terms of description and notof limitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A system for low-current testing of a test device comprising: (a) a probing device for probing a probing site on said test device, said probing device including a dielectric substrate having first and second sides, an elongate conductive path on said first side of said substrate, an elongate probing element connected to said elongate conductive path so as to extend in a cantilevered manner beyond said substrate, and a conductive area on said second side of said substrate; (b) a probe housing matingly detachably engageable with said probing device; (c) said probe housing being engaged with both a first cable that includes a first conductor surrounded by a second conductor and a second cable including a third conductor surrounded by a fourth conductor; (d) a first conductive path that electrically interconnects said first conductor, said third conductor, and said elongate conductive path on said first side of said substrate when said probing device is engaged with said probe housing; and (e) a second conductive path that electrically interconnects said second conductor and said fourth conductor with said conductive area on said second side of said substrate when said probing device is engaged with said probe housing, where said first conductive path is electrically isolated from said second conductive path.
 2. The system of claim 1 wherein said probe housing further includes an elongate opening in which said probing device is matingly detachably engageable therewith.
 3. The system of claim 2 wherein said opening defines a first upright surface therein.
 4. The system of claim 3 wherein said conductive area is in face-to-face contact with said first upright surface when said probing device is engaged with said probe housing.
 5. The system of claim 4 wherein said system further comprises an insert within said probe housing that includes an insulator in face-to-face abutment with a second upright surface opposing said upright surface and a conductive member that is in connection with said elongate conductive path.
 6. The system of claim 5 wherein said conductive member is resiliently deformable as said probing device is said detachably engageable with said probing device.
 7. A system for low-current testing of a test device comprising: (a) a probing device for probing a probing site on said test device, said probing device including a dielectric substrate having first and second sides, an elongate conductive path on said first side of said substrate, an elongate probing element connected to said elongate conductive path so as to extend in a cantilevered manner beyond said substrate, and a conductive area on said second side of said substrate; (b) a probe housing matingly detachably engageable with said probing device; (c) said probe housing being detachably engageable with both a first cable that includes a first conductor surrounded by a second conductor and a second cable that includes a third conductor surrounded by a fourth conductor; (d) a first conductive path that electrically interconnects said first conductor, said third conductor, and said elongate conductive path on said first side of said substrate when said probing device is engaged with said probe housing, said first cable is engaged with said probe housing, and said second cable is engaged with said probe housing; and (e) a second conductive path that electrically interconnects said second conductor and said fourth conductor with said conductive area on said second side of said substrate when said probing device is engaged with said probe housing.
 8. The system of claim 7 wherein said probe housing further includes an elongate opening in which said probing device is matingly detachably engageable therewith.
 9. The system of claim 8 wherein said opening defines a first upright surface therein.
 10. The system of claim 9 wherein said conductive area is in face-to-face contact with said first upright surface when said probing device is engaged with said probe housing.
 11. The system of claim 10 wherein said system further comprises an insert within said probe housing that includes an insulator in face-to-face abutment with a second upright surface opposing said upright surface and a conductive member that is in connection with said elongate conductive path.
 12. The system of claim 11 wherein said conductive member is resiliently deformable as said probing device is said detachably engageable with said probing device.
 13. A probe housing for holding a test device comprising: (a) said probe housing matingly detachably engageable with a probing device for probing a probing site on said test device that includes a dielectric substrate having first and second sides, an elongate conductive path on said first side of said substrate, an elongate probing element connected to said elongate conductive path so as to extend in a cantilevered manner beyond said substrate, and a conductive area on said second side of said substrate; (b) said probe housing being detachably engageable with both a first cable that includes a first conductor surrounded by a second conductor and a second cable that includes a third conductor surrounded by a fourth conductor; (c) a first conductive path that electrically interconnects said first conductor, said third conductor, and said elongate conductive path on said first side of said substrate when said probing device is engaged with said probe housing, said first cable is engaged with said probe housing, and said second cable is engaged with said probe housing; and (d) a second conductive path that electrically interconnects said second conductor and said fourth conductor with said conductive area on said second side of said substrate when said probing device is engaged with said probe housing.
 14. The system of claim 13 wherein said probe housing further includes an elongate opening in which said probing device is matingly detachably engageable therewith.
 15. The system of claim 14 wherein said opening defines a first upright surface therein.
 16. The system of claim 15 wherein said conductive area is in face-to-face contact with said first upright surface when said probing device is engaged with said probe housing.
 17. The system of claim 16 wherein said system further comprises an insert within said probe housing that includes an insulator in face-to-face abutment with a second upright surface opposing said upright surface and a conductive member that is in connection with said elongate conductive path.
 18. The system of claim 17 wherein said conductive member is resiliently deformable as said probing device is said detachably engageable with said probing device.
 19. The system of claim is wherein said first upright surface is flat.
 20. A system for low-current testing of a test device comprising: (a) a probing device for probing a probing site on said test device, said probing device including a dielectric substrate having first and second sides, an elongate conductive path on said first side of said substrate, an elongate probing element connected to said elongate conductive path so as to extend in a cantilevered manner beyond said substrate, and a conductive area on said second side of said substrate; (b) a probe housing matingly detachably engageable with said probing device; (c) said probe housing being detachably engageable with at least a first cable that includes a first conductor surrounded by a second conductor; (d) a first conductive path electrically interconnects said first conductor and said elongate conductive path on said first side of said substrate when said probing device is engaged with said probe housing, and said first cable is engaged with said probe housing; and (e) a second conductive path electrically interconnects said second conductor with said conductive area on said second side of said substrate when said probing device is engaged with said probe housing, where said first conductive path is electrically isolated from said second connective path.
 21. The system of claim 20 wherein said probe housing further includes an elongate opening in which said probing device is matingly detachably engageable therewith.
 22. The system of claim 21 wherein said opening defines a first upright surface therein.
 23. The system of claim 22 wherein said conductive area is in face-to-face contact with said first upright surface when said probing device is engaged with said probe housing.
 24. The system of claim 23 wherein said system further comprises an insert within said probe housing that includes an insulator in face-to-face abutment with a second upright surface opposing said upright surface and a conductive member that is in connection with said elongate conductive path.
 25. The system of claim 24 wherein said conductive member is resiliently deformable as said probing device is said detachably engageable with said probing device.
 26. The system of claim 20 further comprising: (a) said probe housing being detachably engageable with a second cable that includes a third conductor surrounded by a fourth conductor; (b) said first coupler electrically interconnects said third conductor and said elongate conductive path on said first side of said substrate when said probing device is engaged with said probe housing and said second cable is engaged with said probe housing; and (c) said second coupler electrically interconnects said fourth conductor with said conductive area on said second side of said substrate when said probing device is engaged with said probe housing. 